Abstract
Lignocellulosic biomass is an abundant renewable source of chemicals and fuels. Lignin, one of biomass main structural components being widely available as by-product in the pulp and paper industry and in the process of second generation bioethanol, can provide phenolic and aromatic compounds that can be utilized for the manufacture of a wide variety of polymers, fuels, and other high added value products. The effective depolymerisation of lignin into its primary building blocks remains a challenge with regard to conversion degree and monomers selectivity and stability. This review article focuses on the state of the art in the liquid phase reductive depolymerisation of lignin under relatively mild conditions via catalytic hydrogenolysis/hydrogenation reactions, discussing the effect of lignin type/origin, hydrogen donor solvents, and related transfer hydrogenation or reforming pathways, catalysts, and reaction conditions.
Highlights
The projected depletion of fossil fuels and the deterioration of environment by their intensive use has fostered research and development efforts towards utilization of alternative sources of energy.Biomass from non-edible crops and agriculture/forestry wastes or by-products is considered as a promising feedstock for the replacement of petroleum, coal, and natural gas in the production of chemicals and fuels
The aim of this review is to focus on the heterogeneous catalytic transfer hydrogenation reactions for the depolymerization of various types of lignins, including technical lignins deriving from established industrial processes, i.e., kraft, soda or lignosulphonate lignin from the pulp and paper or related industries, as well as enzymatic/acid hydrolysis and organosolv lignins as part of the 2nd
The valorization of lignin, being the most abundant natural phenolic/aromatic polymer, has tremendous potential provided that efficient depolymerization processes will be developed
Summary
The projected depletion of fossil fuels and the deterioration of environment by their intensive use has fostered research and development efforts towards utilization of alternative sources of energy. Lignocellulosic biomass consists mainly of cellulose, hemicellulose and lignin, all of which can be converted into a wide variety of platform chemicals that can be further transformed to fuels, engineering polymers, pharmaceuticals, cosmetics, etc. A number of pretreatment methods have been proposed for the selective isolation of each biomass component These include physical methods such milling [5,6,7], sometimes combined with H2 SO4 , chemical methods such as acid (H2 SO4 , HCl, H3 PO4 ), alkaline. [18,19,20] the downstream selective depolymerization of these biopolymers to their primary building units, i.e., glucose, xylose, alkoxy-phenols, etc., and their consequent transformation to a wide variety of platform chemicals and eventually to final products, may offer even higher value to biomass valorization, via the “biorefinery”. Reproduced from reference [27] with permission from MDPI
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